The invention relates generally to insulated concrete wall panels and, more specifically, to insulated concrete wall panels having tendons or rods that lie in the plane of the insulation and are placed under tension after the concrete panel has been cast.
Insulated precast concrete wall panels are well known in the art of building construction. Two general methods of panel fabrication are used, site-cast construction, in which the panels are fabricated horizontally at the building site and where they are subsequently erected, and plant-cast construction, in which the panels are fabricated horizontally at a fixed, remote plant and are shipped to the building site for erection.
A wall panel, at a minimum, must be capable of resisting forces applied both normal to and in its plane. Normal forces can result from environmental effects, such as wind, and geologic effects, such as from earthquakes. In-plane forces can also result from wind or earthquake, but are imposed on the wall panels through connections with the horizontal elements in the building, including horizontal roof bracing or diaphragms created by roof or floor systems. The weight of the panel will create in-plane forces, and, in many cases, a wall may also be required to carry superimposed gravity loads from roof or floor structures. Although columns that are independent from the walls could be used to carry these gravity loads, it is often economical—both in material and floor space—if the walls are used to support the perimeter of the roof or floor structures in lieu of an exterior colonnade.
Additional forces that must be considered in the design of wall panels include forces imposed during handling and erection of the panels, as well as internal forces created by temperature and shrinkage differentials that occur after the panel is erected.
Because concrete is itself a relatively brittle material with a low tensile capacity, it must be reinforced with a material capable of carrying large tensile strains without fracture. Although fiber composite materials can be used, the most common reinforcing material used in wall construction is steel. Regardless of the material used, the stress in the reinforcing at the time of fabrication defines still more subsets of wall construction types.
These additional types of wall construction are, in general, reinforced concrete and prestressed concrete. In reinforced concrete, the initial strain in the steel is essentially equal to the initial strain in the concrete. The steel is placed in forms, followed by plastic concrete. When the concrete hardens sufficiently, the panel is ready for handling and erection, where the steel and the concrete are subjected to both tensile and compression strains. In contrast, even before handling, prestressed concrete is fabricated such that the strain in the steel is tensile and the strain in the concrete is compressive. The pre-imposed compressive strain in the concrete has a number of advantages, which will be discussed in more detail elsewhere in this specification. It is of interest to note that the shrinkage of concrete after it sets actually creates tensile strains in the concrete as well as compressive strains in the steel.
Within the classification of prestressed concrete, two further subsets of pre-tensioned and post-tensioned construction exist. These subsets are defined by the sequence and the methods used to prestress the reinforcing material and to transfer compressive stresses into the concrete. In pre-tensioned construction, the reinforcing material is placed in tension by jacking against a relatively stiff form or bed. The form or bed therefore supplies the reaction necessary to pull the reinforcing material. While the form or bed is therefore placed in compression, the strain in the bed has only a minor effect on the final wall panel itself. Plastic concrete is placed around the pre-tensioned steel and is allowed to cure and harden. When the concrete has reached a sufficient compressive strength to survive the imposition of compressive and flexural stresses imposed by the action, the external restraint is removed from the reinforcing. The reinforcing therefore shortens and imposes significant compression strains in the concrete itself, usually through bond between the concrete and the steel.
In post-tensioned construction, the initial construction sequence and therefore the initial strain in the concrete is nearly the same as those for reinforced concrete. The one major exception is that some or much of the reinforcing is isolated from contact with the plastic concrete. After the concrete has hardened and reached sufficient strength, this isolation allows the reinforcing to be tensioned by using the concrete member itself to supply the reaction. In this case, the concrete within the panel is placed in compression, and this compression is maintained by placing an anchorage that allows the tension in the reinforcing to be transferred as a compressive reaction at each end of the post-tensioned reinforcing.
Although tensile forces can arise from all sources cited above (including eccentrically applied prestressing), tensile forces resulting from flexural loads imposed by wind or earthquake as well as internal forces arising from temperature effects are usually the most significant.
Post-tensioning systems are well known in the art. Systems that incorporate threaded tension members are, for example, supplied by Dywidag and Williams Form Engineering. The Dur-O-Wall® Sure-Stress™ post-tensioning system is an example of similar systems used in masonry construction, and includes the use of steel rods and load-indicating washers.
One form of direct tension indicating washers are described in U.S. Pat. No. 5,931,618. These washers include indicating material, normally silicone, which is positioned in indentations within each washer. As the nut is turned on the bolt or, in this case, the post-tensioning tendon or rod, the tendon is placed in tension and the washer is compressed. As the indentations are flattened, channels formed from the indentations to the outside diameter of the washer allow the indicating material to migrate to the outer diameter of the washer. Sufficient washer compression and resulting rod tension are indicated when a designated number of channels have carried the indicating material to the exposed perimeter of the washer.
Lifting inserts and devices are well known in the art. The inserts are normally cast in the concrete layers and are loaded using proprietary lifting clutches that, in turn, connect to wire ropes. Examples of lifting devices that can be used with threaded rods are also known in the art. The Dayton/Richmond Swivel Lifting Plate consists of a heavy steel casting and a drop forged bail that is pinned to allow a full 180° swivel.